1. Field
The disclosed embodiments relate to headset switch press detection.
2. Background Information
Cellular telephones often have a headset/microphone accessory. The headset/microphone accessory typically involves two speakers, one for the left ear and one for the right ear. The speakers are usable to listen to audio that is output from the cellular telephone. The headset/microphone accessory also has a microphone. Rather than speaking into the microphone on the body of the cellular telephone, the user can speak into the microphone on the accessory. The accessory is therefore useful for hands free operation of the cellular telephone.
As the user speaks into the microphone on the accessory, the effective resistance of the microphone varies. A resistor of fixed resistance is placed in series with the microphone to form a voltage divider so that the changing microphone resistance is converted into a changing voltage between ground potential and a voltage on the center tap of the voltage divider. The changing voltage is the audio input signal that is detected by an integrated circuit within the cellular telephone. The integrated circuit includes a voltage regulator that supplies a regulated voltage onto a terminal called the microphone bias (MIC BIAS) terminal of the integrated circuit. The voltage divider is coupled to the MIC BIAS terminal so that the regulated MIC BIAS voltage properly biases the voltage divider.
In addition to being usable to convert sound into an audio input signal, the headset/microphone accessory may also include a pushbutton switch. The cell phone user can press the pushbutton switch to send a control signal to cellular telephone. The function of the control signal typically depends on the operating mode of the cellular telephone. For example, if the cellular telephone rings, some cellular telephones allow the user to pick up the call by pressing the pushbutton switch. When the pushbutton switch is pressed, the switch effectively shorts out the microphone. Although speech from the user is not being converted into a usable audio input signal during the switch press condition, this is acceptable because the headset/microphone accessory is being used to communicate the control signal to cellular telephone. When the switch is pressed, an unusually large amount of current is drawn out of the MIC BIAS terminal for an unusually large amount of time. This high current condition is detected by the integrated circuit as a switch press condition. This high current condition is converted into a high voltage condition which in turn can be detected by an on-chip analog-to-digital converter (ADC). The cell phone may, for example, interpret the switch press condition as a desire to pick up the call and respond accordingly.
When the cellular telephone is not being used to make a call, the integrated circuit is powered down into a low-power sleep state. This low power sleep state is maintained for a large proportion of time in order to reduce power consumption of the cellular telephone. Periodically, the cellular telephone momentarily wakes up into a higher power state to check to see if there is an incoming call. Presuming there is no call, the cellular telephone returns to the sleep state. In this way, the integrated circuit that includes the voltage regulator and the ADC is periodically powered up in order to check for an incoming call, but is otherwise kept in the low power sleep mode.
In order for the cellular telephone to detect a switch press condition, however, the MIC BIAS voltage regulator and the ADC must be powered and functional. The voltage regulator and ADC circuits are, however, not powered and not functional when the cellular telephone is in its low power sleep mode. Accordingly, if a switch detect operation is to be performed, then either there must be a wait period until the end of the current sleep mode cycle, or the integrated circuit must be powered up momentarily and periodically just to do switch detect operation.
For example, FIG. 2 shows a simplified diagram of a cellular telephone 201 that has a headset/microphone accessory 202 plugged into a jack 203 on the cellular telephone body. Voltage regulator 204 within analog integrated circuit 205 supplies a MIC BIAS voltage onto MIC BIAS terminal 206. This voltage biases the microphone of headset/microphone accessory 202. When pushbutton 207 is pressed, an unusually large current is drawn out of MIC BIAS terminal 206. This condition is detected using current mirror 208, resistor 209, and analog-to-digital converter (ADC) 210. When the voltage regulator, current mirror and ADC are unpowered in a low-power sleep mode of integrated circuit 205, the circuit of FIG. 2 cannot detect a switch press condition.
An alternative is desired whereby the switch detect operation can be carried out even when cellular telephone and the integrated circuit are in the low power sleep state.